Determining structural tomographic properties of a geologic formation

ABSTRACT

A variable-frequency light source is configured to emit a light beam and modulate a frequency of the light beam. A fiber optic cable is attached to the variable frequency light source. The fiber optic cable is configured to receive the light beam at an inlet and pass the light beam to an exit. Multiple optical detectors are attached to the fiber optic cable. Each of the optical detectors is configured to detect a specified frequency of light that is backscattered through the fiber optic cable. An actuation mechanism is attached to the fiber optic cable. The actuation mechanism is configured to deform the fiber optic cable in response to a stimulus.

TECHNICAL FIELD

This disclosure relates to sensors in boreholes.

BACKGROUND

Acoustic detection using optical fibers exploits Rayleighback-scattering along a fiber from a short pulse of light traversing thefiber. The back-scattering characteristics change if the fiber is movedor deformed even on the micron-scale. Thus, a record of changes inscattering intensity as a function of pulse transit time correspond tomovement of the fiber as a function of position along the fiber. Thisresults in a distributed acoustic sensor that can be used for welldiagnostics in the oil field on either a temporary or even permanentinstalled basis. Such fiber based sensors can be used to detect theacoustic signature of small leaks, flows, and bubbles within a wellboreor borehole.

SUMMARY

This disclosure relates to determining structural tomographic propertiesof a geologic formation.

An example implementation of the subject matter described within thisdisclosure is a sub-surface detection device with the followingfeatures. A variable-frequency light source is configured to emit alight beam and modulate a frequency of the light beam. A fiber opticcable is attached to the variable frequency light source. The fiberoptic cable is configured to receive the light beam at an inlet and passthe light beam to an exit. Multiple optical detectors are attached tothe fiber optic cable. Each of the optical detectors is configured todetect a specified frequency of light that is backscattered through thefiber optic cable. An actuation mechanism is attached to the fiber opticcable. The actuation mechanism is configured to deform the fiber opticcable in response to a stimulus.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.Each of the optical detectors is configured to detect a differentfrequency of light.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.The actuation mechanism includes an antenna configured to detect changesin an electromagnetic field. An electromechanical transducer isconnected to the antenna. The electromechanical transducer is configuredto deform the fiber optic cable in response to detecting changes in theelectromagnetic field.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.The electromechanical transducer includes a piezoelectric element.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following. Afiltering circuit is configured to filter out noise detected by theantenna. An amplifier is configured to amplify the filtered signal. Anenergy storage device is configured to supply power to at least one ofthe amplifier or the filtering circuit.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.The energy storage device includes a battery or capacitor.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.The actuation mechanism includes a hydrophilic gel configured to expandin the presence of moisture. A diaphragm is connected to the hydrophilicgel and the fiber optic cable. An expansion in the hydrophilic gelinduces the diaphragm to deform the fiber optic cable.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.An energy harvesting device is configured to harvest energy from anambient environment. An energy storage device is configured to store theharvested energy.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.The energy harvesting device includes an antenna configured to harvestelectromagnetic energy within the borehole. A rectification circuit iscoupled to the antenna. The rectification circuit is configured tocondition the harvested energy for storage. An energy storage device iscoupled to the rectification circuit. The energy storage device isconfigured to store the harvested energy.

Aspects of the example implementation, which can be combined with theexample implementation alone or in combination, include the following.The energy harvesting device includes a piezoelectric device configuredto harvest mechanical energy within the borehole. A rectificationcircuit is coupled to the piezoelectric device. The rectificationcircuit is configured to condition the harvested energy for storage. Anenergy storage device is coupled to the rectification circuit. Theenergy storage device is configured to store the harvested energy.

An example implementation of the subject matter described within thisdisclosure is a method with the following features. A modulatingfrequency light beam is directed with a variable frequency light sourcealong a fiber optic cable placed within a borehole. A deformation isinduced in the fiber optic cable with a subsurface detection device thatis responsive to a specified stimulus. A backscatter of multiplefrequencies within the fiber optic cable is detected with multipleoptical detectors attached to the fiber optic cable. Each of the opticaldetectors is configured to detect a specified frequency of light. Aphysical property of a geologic formation in which the borehole isformed is determined in response to detecting the backscatter ofmultiple frequencies.

Aspects of the example method, which can be combined with the examplemethod alone or in combination, include the following. Inducing adeformation in the fiber optic cable includes detecting a change in anelectromagnetic field with an antenna. The deformation within the fiberoptic cable is induced with a transducer coupled to the antenna inresponse to the change in the electromagnetic field.

Aspects of the example method, which can be combined with the examplemethod alone or in combination, include the following. Inducing adeformation in the fiber optic cable includes expanding a hydrophilicgel in response to a moisture content within the borehole. Thedeformation within the fiber optic cable is induced with the expandedhydrophilic gel.

Aspects of the example method, which can be combined with the examplemethod alone or in combination, include the following. Energy isharvested from changes in magnetic fields within the borehole with anantenna. The harvested energy is stored within an energy storage device.Equipment within the borehole is powered with the stored energy.

Aspects of the example method, which can be combined with the examplemethod alone or in combination, include the following. Energy isharvested from ambient noise within the borehole with a transducer. Theharvested energy is stored within an energy storage device. Equipmentwithin the borehole is powered with the stored energy.

Aspects of the example method, which can be combined with the examplemethod alone or in combination, include the following. The energystorage device includes either a super capacitor or a battery.

An example implementation of the subject matter described within thisdisclosure is a distributed geophone system with the following features.Multiple sub-surface detection devices are installed within a pluralityof boreholes. Each of the sub-surface detection devices includes avariable frequency light source configured to emit a light beam andmodulate a frequency of the light beam. A fiber optic cable is attachedto the variable frequency light source. The fiber optic cable isconfigured to receive the light beam. Multiple optical detectors areattached to the fiber optic cable. Each of the optical detectors isconfigured to detect a specified frequency of light that isbackscattered through the fiber optic cable. An actuation mechanism isattached to the fiber optic cable. The actuation mechanism is configuredto deform the fiber optic cable in response to a stimulus. A centralprocessing center is configured to monitor and store data produced bythe sub-surface detection devices.

Aspects of the example system, which can be combined with the examplesystem, alone or in combination, include the following. The actuationmechanism includes an antenna configured to detect changes in anelectromagnetic field. An electromechanical transducer is connected tothe antenna. The electromechanical transducer is configured to deformthe fiber optic cable in response to detecting changes in theelectromagnetic field.

Aspects of the example system, which can be combined with the examplesystem, alone or in combination, include the following. The actuationmechanism includes a hydrophilic gel configured to expand in thepresence of moisture. A diaphragm is connected to the hydrophilic geland the fiber optic cable. An expansion in the hydrophilic gel inducesthe diaphragm to deform the fiber optic cable.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description below. Other features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an example borehole sensor systeminstalled within a borehole.

FIG. 2A shows a schematic diagram of an example moisture sensor.

FIG. 2B shows a schematic diagram of an example of an electromagneticsensor.

FIG. 3 shows a schematic diagram of a downhole sensor system.

FIG. 4 is a flowchart of an example method that can be used with certainaspects of this disclosure.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Optical fibers have some limitations in regards to their use in sensors.First, detection is based exclusively on mechanical deflection of thefiber. Second, the maximum useful frequency is half of the sampling rate(Nyquist frequency) in this system and depends on the length of thefiber, or more specifically, the round trip transit time of the lightfrom start to end and back. For example, about 6 ns per meter of length.A typical horizontal well can be about 5 km, resulting in 30,000 ns oftravel time, or a sample rate of 33 kHz. A 50 km pipeline applicationwould be limited to about a 3 kHz sampling rate. Aspects of thisdisclosure enable long-term, low-cost detection of acoustic/ultrasonic,electric and magnetic fields at much higher frequencies than currenttechnology.

The disclosure is directed to extending and adapting current opticalfiber distributed acoustic sensors (DAS) to frequency multiplexing.Frequency multiplexing can use multiple light sources with each havingdifferent frequency bands. Typically, frequency multiplexing canincrease effective sampling rates, and thereby increase bandwidth. Forthe frequency multiplexing to increase bandwidth, instead of using asingle frequency laser to probe the state of a distributed fiber sensor,multiple different laser frequencies are used to probe the state of thedistributed fiber sensor in parallel. With frequency multiplexing,sampling rate can be increased by several times and enabling distributedsensing of radio frequencies or ultrasonic signals. The fiber opticcable can act as a data collector from multiple sensors mounted alongthe well path, either behind tubing, behind casing, or run in hole as anintervention based system. The sensors can have various characteristicsdepending on the purpose of the sensor. For sensing electric or magneticfields, separate antennas and transducers are affixed to the side of thefiber, with the antennas receiving electric signals, magnetic signals,or both. The transducers impart a detectable, mechanical deflection tothe fiber, while the DAS can provide distributed electromagneticsensing. In addition to the electromagnetic sensing, changing moisturewithin a borehole can be detected as well. In some implementations, asuitable energy storage and energy harvesting system can be supplied toincrease a deployable lifetime of the system.

FIG. 1 is a schematic diagram of an example sub-surface detection device100 installed within a borehole 102 formed within a geologic formation104. In some implementations, multiple sub-surface detection devices areinstalled within multiple boreholes to form a distributed geophonesystem. The sub-surface detection device 100 includes a variablefrequency light source 106 that is configured to emit a light beammodulate at a specified frequency. A fiber optic cable 108 is attachedto the variable frequency light source 106. The fiber optic cable 108 isconfigured to receive the light beam and guide the light beam along itslength. Multiple optical detectors 110 are attached to the fiber opticcable 108 and are configured to detect backscattered light. In someimplementations, each of the multiple optical detectors 110 isconfigured to detect a specified frequency of light that isbackscattered through the fiber optic cable 108. In someimplementations, each of the multiple optical detectors 110 isconfigured to detect a different frequency or frequency band of light.An actuation mechanism 112 is attached to the fiber optic cable 108. Theactuation mechanism 112 is configured to deform, deflect, or otherwisedisplace the fiber optic cable 108 in response to a stimulus, which, inturn, can backscatter light. The multiple optical detectors 110 candetermine the displacement based on the backscatter. In someimplementations, a central processing center 114 is configured tomonitor and store data produced by one or more sub-surface detectiondevices.

The fiber optic cable 108 can accommodate a broad band of frequencies.As a result, sweeping (modulating) the laser frequency combined with themultiple optical detectors 110 can separately determine thebackscattered intensity at each frequency (or specified frequency band)separately. In addition, the modulation of the laser frequency as afunction of time also allows one to effectively multiplex “N” differentlaser probes in parallel (where “N” is the total sweep band divided bythe width of each detection band). Such multiplexing can increase a DASdetection bandwidth by several hundred times. To optimize or otherwiseincrease readout speed, an array of single channel detectors, interfacedvia optical fibers, with a number of single channel digitizers operatingin parallel can be used in some implementations. Signal processingcomparing current to prior backscatter data can be performed withcustomized integrated circuits. By way of example, using the 5 km wellmentioned above, a 100× increase in throughput using frequencymultiplexing would increase the effective sampling rate to 33×100 kHz,or 3.3 MHz, which is sufficient for distributed sensing of RF orultrasonic signals up to about 1 MHz.

FIG. 2A illustrates an example of a moisture sensor that can be utilizedwith aspects of this disclosure. In the illustrated example, theactuation mechanism 112 includes a hydrophilic gel 202 configured toexpand in the presence of moisture. A diaphragm 204 is connected to thehydrophilic gel 202 and the fiber optic cable 108. An expansion in thehydrophilic gel 202 induces the diaphragm 204 to deform the fiber opticcable 108.

FIG. 2B illustrates an example of an electromagnetic sensor andtransducer that can be utilized with aspects of this disclosure, forexample, within the actuation mechanism 112. In the illustratedimplementation, the actuation mechanism 112 includes an antenna 252configured to detect changes in an electromagnetic field 254. Theantenna 252 can include a dipole antenna, a ferrite loopstick, or anyother antenna appropriate for the desired electromagnetic field to bemeasured. An electromechanical transducer 256 is connected to theantenna 252. The electromechanical transducer 256 is configured todeform the fiber optic cable 108 in response to detecting changes in theelectromagnetic field 254. The electromechanical transducer 256 caninclude a piezoelectric element, an electrostatic diaphragm, anelectrical coil with a diaphragm, or other electromechanical devices.

In some implementations, multiple actuation mechanisms 112 can be usedon a single fiber optic cable 108. In such an implementation, actuationmechanisms 112 can be used on the same line. For example, anelectromagnetic sensor and a moisture sensor can be used on the sameline. In some implementations, multiples of the same sensor type can beused. For example, multiple electromagnetic sensors can be installed onthe same fiber optic line.

FIG. 3 shows a schematic diagram of a downhole sensor system 300. Inaddition to the variety of components previously described, the downholeactuation mechanism can include a filtering circuit 302 configured tofilter out noise detected by the antenna 252. Additionally, an amplifier304 can be included downhole to amplify the filtered signal. Theadditions of the filtering circuit 302 and the amplifier 304 increasesthe sensitivity and accuracy of the actuation mechanism 112. In someimplementations, an energy storage device 306 can be included to supplypower to the amplifier 304, the filtering circuit 302, or both. Theenergy storage device 306 can include a battery, a capacitor, or anyother energy storage device.

In some implementations, the subsurface detection device 300 includes anenergy harvesting device 308 configured to harvest energy from anambient environment and store the energy within the energy storagedevice 306 configured to store the harvested energy. In someimplementations, the harvesting device 308 can include an antennaconfigured to harvest electromagnetic energy within the borehole. Insome implementations, the antenna 252 shown in FIG. 2B can be used forboth harvesting energy and actuating the electromechanical transducer256. In such an implementation, a rectification circuit 310 is coupledto the antenna. The rectification circuit 310 is configured to rectifyand otherwise condition the harvested energy for storage. For example,the rectification circuit 310 can convert a noisy, oscillating currentinto a constant, DC current that can be used to charge a battery. Insuch an implementation, an energy storage device 306 is coupled to therectification circuit 310, and is configured to store the harvestedenergy. In some implementations, the energy storage device 306 includesa super capacitor, a battery, or any other energy storage mechanism.

In some implementations, the harvesting device 308 can include anelectromechanical transducer to harvest noise energy within theborehole. In some implementations, the actuation mechanism 112 shown inFIG. 2B can be used for both harvesting energy and deforming the opticalcable 108. The electromechanical transducer can include a piezoelectricdevice, an electrostatic transducer, an electrical coil and diaphragm,or any other electromechanical transducer. In such an implementation,the rectification circuit 310 is coupled to the electromechanicaltransducer. The rectification circuit 310 is configured to rectify andotherwise condition the harvested energy for storage. For example, therectification and circuit can convert a noisy, oscillating current intoa constant, DC current that can be used to charge a battery. In such animplementation, the energy storage device 306 is coupled to therectification circuit 310, and is configured to store the harvestedenergy. In some implementations, the energy storage device 306 includesa super capacitor, a battery, or any other energy storage mechanism.

FIG. 4 is a flowchart of an example method 400 that can be used withaspects of this disclosure. At 402, a light beam with a modulatingfrequency is directed with a variable frequency light source along afiber optic cable 108 placed within a borehole. At 404, a deformation isinduced in the fiber optic cable 108 with a subsurface detection devicethat is responsive to a specified stimulus, for example, the previouslydescribed moisture sensor in FIG. 2A and electromagnetic sensor in FIG.2B. In some implementations, different subsurface detection devices cancause backscatter at specified frequencies. In some implementations,multiple actuation mechanisms 112 can be included on a single fiberoptic cable. At 406, the backscatter of multiple frequencies is detectedwithin the fiber optic cable by the multiple optical detectors 110attached to the fiber optic cable. Each of the multiple opticaldetectors 110 is configured to detect a specified frequency of light. Insome implementations, each of the multiple optical detectors 110 isconfigured to detect changes in a different frequency band. At 408, aphysical property of a geologic formation in which the borehole isformed is determined in response to detecting the backscatter ofmultiple frequencies. For example, a change in moisture content can bedetermined by detecting a backscatter in a first frequency, while achange in an electromagnetic field can be determined by detecting abackscatter in a second frequency.

In implementations where an electromagnetic sensor is used, a change inan electromagnetic field is detected with the antenna 252. A current isinduced in the antenna 252 by the changing magnetic fields. Theelectromechanical transducer 256 is actuated in response to the inducedcurrent. The fiber optic cable 108 is deformed by the electromechanicaltransducer 256. As previously described, the electromechanicaltransducer 256 can include a piezoelectric element, an electrostaticdiaphragm, an electrical coil with a diaphragm, or any otherelectromechanical device.

In implementations where a moisture sensor is used, a hydrophilic gelexpands in response to a moisture content within the borehole. Thedeformation within the fiber optic cable is induced when the expandedhydrophilic gel deforms a diaphragm 204 that is attached to the fiberoptic cable 108. The diaphragm can include an elastomer that isappropriate for service within a wellbore. That is, the elastomer ischemically inert and resistant to temperatures found within the borehole102. For example, Teflon, Viton, or any other inert elastomer can beused.

In implementations where electromagnetic energy is harvested fromchanges in magnetic fields within the borehole, the energy is harvestedwith an antenna, such as antenna 252. In such an instance, the harvestedenergy is conditioned and stored within the energy storage device 306.Downhole equipment, such as the actuation mechanism 112, can be poweredwith the stored energy.

In implementations where energy from ambient noise within the boreholeis harvested, the energy is harvested with a transducer, thenconditioned and stored within the energy storage device 306. Downholeequipment, such as the actuation mechanism 112, can be powered with thestored energy.

Regardless of what energy harvesting mechanism is used, the energystorage device 306 in which the harvested energy is stored can be abattery, a super capacitor, or any other energy storage device.

While this disclosure contains many specific implementation details,these should not be construed as limitations on the scope of what may beclaimed, but rather as descriptions of features specific to particularimplementations. Certain features that are described in this disclosurein the context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theimplementations described above should not be understood as requiringsuch separation in all implementations, and it should be understood thatthe described components and systems can generally be integratedtogether in a single product or packaged into multiple products.

Thus, particular implementations of the subject matter have beendescribed. Other implementations are within the scope of the followingclaims. In some cases, the actions recited in the claims can beperformed in a different order and still achieve desirable results. Inaddition, the processes depicted in the accompanying figures do notnecessarily require the particular order shown, or sequential order, toachieve desirable results.

What is claimed is:
 1. A sub-surface detection device comprising: avariable-frequency light source configured to emit a light beam andmodulate a frequency of the light beam; a fiber optic cable attached tothe variable frequency light source, the fiber optic cable configured toreceive the light beam at an inlet and pass the light beam to an exit; aplurality of optical detectors attached to the fiber optic cable, eachof the optical detectors configured to detect a specified frequency oflight that is backscattered through the fiber optic cable; and anactuation mechanism attached to the fiber optic cable, the actuationmechanism configured to deform the fiber optic cable in response to astimulus; wherein the actuation mechanism comprises: a hydrophilic gelconfigured to expand in the presence of moisture; and a diaphragmconnected to the hydrophilic gel and the fiber optic cable, wherein anexpansion in the hydrophilic gel induces the diaphragm to deform thefiber optic cable.
 2. The sub-surface detection device of claim 1,wherein each of the plurality of optical detectors is configured todetect a different frequency of light.
 3. The sub-surface detectiondevice of claim 1, wherein the actuation mechanism comprises: an antennaconfigured to detect changes in an electromagnetic field; and anelectromechanical transducer that is connected to the antenna, theelectromechanical transducer configured to deform the fiber optic cablein response to detecting changes in the electromagnetic field.
 4. Thesub-surface detection device of claim 3, wherein the electromechanicaltransducer comprises a piezoelectric element.
 5. The subsurfacedetection device of claim 3, further comprising: a filtering circuitconfigured to filter out noise detected by the antenna; an amplifierconfigured to amplify the filtered signal; and an energy storage deviceconfigured to supply power to at least one of the amplifier or thefiltering circuit.
 6. The subsurface detection device of claim 5,wherein the energy storage device comprises a battery or capacitor. 7.The subsurface detection device of claim 6, wherein the energyharvesting device comprises: an antenna configured to harvestelectromagnetic energy within the borehole; a rectification circuitcoupled to the antenna, the rectification circuit configured tocondition the harvested energy for storage; and an energy storage devicecoupled to the rectification circuit, the energy storage deviceconfigured to store the harvested energy.
 8. The subsurface detectiondevice of claim 6, wherein the energy harvesting device comprises: apiezoelectric device configured to harvest mechanical energy within theborehole; a rectification circuit coupled to the piezoelectric device,the rectification circuit configured to condition the harvested energyfor storage; and an energy storage device coupled to the rectificationcircuit, the energy storage device configured to store the harvestedenergy.
 9. The subsurface detection device of claim 1, furthercomprising: an energy harvesting device configured to harvest energyfrom an ambient environment; and an energy storage device configured tostore the harvested energy.